The invention generally relates to processing a seismic monitor survey.
Much effort is expended in locating, evaluating, and exploiting hydrocarbon deposits, e.g., oil and natural gas, trapped in subterranean geological formations. It is highly desirable to locate hydrocarbon deposits in reservoirs. For present purposes, a “reservoir” shall be considered to be any geological medium containing voids (e.g., pores or fractures) in the subsurface from which liquid or gas can be extracted or into which liquid or gas can be injected. However, such reservoirs can be exploited in a number of ways other than extracting hydrocarbons. For instance, such reservoirs can also be used to temporarily store hydrocarbons previously produced or for carbon dioxide (“CO2”) sequestration. Once located, a reservoir may be evaluated for potential production or other exploitation. A likely prospect can then be exploited by, for instance, drilling a well through which or into which a gas or a liquid can be extracted or injected.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected waves, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
An important step in obtaining a subsurface image from seismic data is a process called migration. One key prerequisite for this process is the availability of a suitable velocity model. Traditionally, the velocity model is derived from manual or automatic picking of seismic arrival times and suitable processing. This process may be time-consuming and therefore expensive.
In time lapse siesmics, the goal is to derive two or more sets of seismic energies and seismic attributes, which contain information about reservoir properties, such as fluid saturations, stress, pressure or temperature. The first seismic survey is called a base survey, and subsequent surveys are called monitor surveys. Migration of the monitor surveys requires a velocity model. Using conventional techniques, the velocity model that is derived from the base survey is also used for migration of the monitor surveys. Such an approach avoids the time-consuming process of velocity picking and analysis. However, the conventional approach may lead to sub-optimal migration images and may introduce errors in migration amplitudes due to mis-stacking caused by a sub-optimal velocity model.